Thermal barrier coating (TBC) spallation life during service in a gas turbine engine is largely determined by the chemical composition of the substrate and the interaction of the substrate with the coating system. Substrates are typically made of a high temperature metal alloy such as a gamma prime strengthened nickel superalloy or a cobalt-based superalloy. If a given superalloy substrate has a low concentration of aluminum or a high concentration of titanium, or if the majority element of the superalloy is cobalt (alloys such as ECY 768 and X-45), aluminum in a desired bond coat material such as a CoNiCrAlY or NiCoCrAlY alloy may diffuse rapidly into the superalloy, thereby depleting the bond coat and reducing the effective life of the coating system. Due to the requirement for high strength at elevated temperatures in turbine applications, the choice of substrate is often decided on the basis of creep strength, corrosion resistance and fatigue life, rather than on coating compatibility. Cost and manufacturing concerns such as castability and weldability are also prime drivers in alloy selection. As a result, many of the common superalloys used in aero and land-based turbines have compositions that are unfavorable for bond coat compatibility.
Some gas turbines of the present assignee use a superalloy known in the industry as IN-939 for selected components in the hot gas flow path, such as in the first two rows of turbine vanes. These components rely on TBCs to reduce metal temperature to meet the component design life. If the TBC spalls, the component life will be reduced, increasing engine maintenance, part scrap rate, and repair costs. IN-939 has several properties that make it desirable for stationary hot section components, including low cost, good castability, good weldability and excellent fatigue life. However, IN939 has a relatively low aluminum content and a relatively high titanium content, which rapidly depletes the aluminum-rich beta phase of the bond coat as well as diffusing the harmful element titanium into the bond coat, resulting in decreased coating life. Laboratory furnace cycling tests have shown that TBC life on IN-939 is significantly lower than TBC life on substrates made from more coating-compatible known alloys such as Haynes 230, Mar M002, or CM247. Changing from IN-939 to such an alloy that has better coating compatibility would be one means of increasing coating life, but this is often not feasible for reasons of cost or material requirements. For example, Haynes 230 does not possess the high temperature strength of IN-939, and CM247 is more expensive, harder to cast, and more difficult to weld than IN-939. However, both Haynes 230 and CM247 have far superior oxidation resistance compared to IN-939, which is important for component life after TBC spallation.